Intelligent transportation systems, such as ETC systems, use radio frequency (RF) communications between roadside readers and transponders within or attached to vehicles. The readers form part of an automatic vehicle identification system for uniquely identifying vehicles in an area, such as a toll plaza. Each reader emits a coded identification signal, and when a transponder enters into communication range and detects the reader, the transponder sends a response signal. The response signal contains transponder identification information, including a unique transponder ID. In the United States, current ITS-based, and in particular ETC-based, RF communication systems are licensed under the category of Location and Monitoring Systems (LMS) through the provisions of the Code of Federal Regulations (CFR) Title 47 Part 90 Subpart M.
Vehicle-mounted transponders may either be active or passive. Active transponders contain a battery that powers the transponder. Each transponder listens for a trigger pulse or signal from a roadside reader and, upon sensing one, generates and transmits a response signal. Passive transponders rely upon energy supplied by the roadside reader in the form of a continuous wave RF signal. The continuous wave signal energizes the transponder and the transponder transmits its response signal by way of backscatter modulation of the continuous wave signal. Passive transponders may or may not include a battery in some implementations.
In some situations, the road stations are designed to be “open road”, also known as “multi-lane free-flow”, meaning that communications are conducted at highway speed and there are no physical lane separations so vehicles are not constrained. In ETC systems this occurs with no gates, which means that transactions occur quickly, and also means that there is no gate or barrier that prevents a vehicle without a valid transponder from traversing the toll plaza area. Open road ETC systems rely upon ex post facto enforcement. For example, in many implementations an image is captured of each vehicle's license plate area. The image capture depends on a vehicle detection mechanism, such as a light curtain or magnetic loop for detecting vehicle presence in the roadway. The vehicle detection and image capture point is often outside of the RF capture zone within which the vehicle-mounted transponder communicates with the ETC system. The ETC system may be tasked with correlating captured license plate images with processed transponder-based toll transactions to determine whether any of the vehicle license plate images belong to a vehicle that did not complete a successful electronic toll transaction. That vehicle's owner may then be sent an invoice for the toll amount.
In other ITS stations, the station may be measuring vehicle characteristics such as weight, or volume, or speed, and the system is tasked with correlating the instrument measurements with processed transponder-based transactions to associate the measurements to the vehicles. Image capture may also be used in such stations.
The challenge in any open road system is to quickly and accurately correlate vehicle information from sensors, like license plate images, with the transponder communication transaction. In ETC systems it is particularly important that the detected vehicles are correlated with processed toll transactions in order to identify which vehicle, if any, did not pay a toll via a transponder. One of the challenges in all these systems is to accurately estimate the path travelled by a vehicle associated with a transponder that has completed a transaction, so that the vehicle's position can be correlated to the other sensors, e.g. a vehicle identified by the vehicle detection system used by the image capture system.
Similar reference numerals are used in different figures to denote similar components.